High temperature batteries, such as sodium/sulfur batteries, in which the electrolyte-separator takes the form of a large number of hollow fibers are now well known. In discharge of such a sodium/sulfur battery cell, the anodic reactant, molten sodium, gives up electrons to an external circuit and forms Na.sup.+ ions which migrate through the fiber walls to the catholyte. The cathodic reactant, molten sulfur, takes up electrons from the external circuit to form sulfide anions. The catholyte is thus converted to a mixture or solution of sodium polysulfide(s) with or in sulfur.
Hollow fiber batteries of this type, and methods of making them, are described in the following U.S. patents, the disclosures of which are incorporated herein by reference for all purposes which such references may serve:
______________________________________ U.S. PAT. NO. Subject Matter ______________________________________ 3,476,602 Hollow fiber type high-temperature batteries. 3,672,995 Hollow fiber electrolyte-separator materials. 3,679,480 Hollow fiber type electrical cell assembly. 3,703,412 Sequence of melting anolyte and catholyte. 3,749,603 MoS.sub.2 coated foil as current collect- ing means in Na/S cell. 3,765,944 C-coated foil as current collector. 3,791,868 Method of making a battery cell having a coiled foil as a current collector. 3,829,331 Hollow fibers of sodium borate glass. 3,917,490 Method of grinding tubesheet glasses. 4,050,915 Cooling a nascent, melt-spun, hollow glass fiber. ______________________________________
The prior art cells described in the above-listed patents preferably are hermetically sealed, but do not necessarily have to be closed by seals or welds requiring high temperatures for their information. In fact, when such a cell is so insulated and heated that its upper end remains relatively cool during operation, the openings in the upper casing wall through which the leads pass, and/or which function as fill ports, can be sealed with materials such as pitches, tars or high-melting waxes. However, for the majority of commercial applications contemplated for high temperature batteries, it will not be very practicable to maintain different parts of the component cells at different temperatures. For this reason, and by reason of obvious safety concerns, a more permanent and reliable method of sealing, as by welding or use of solder glasses, is indicated.
The latter sealing methods are feasible but, in the rigid structures of the prior art, have been found to result in thermally induced stresses which can damage the tubesheet, the seals themselves and/or some of the fibers, which are relatively fragile. When forming welds or seals between the high melting or refractory materials of construction most suitable for high temperature battery cells, it is difficult to achieve a high degree of cross-sectional uniformity and to attain even cooling of the seals. Consequently, dimensional changes resulting in shifting of the inner assembly (anolyte cup, tubesheet, fibers and foil wraps; see drawings) within the casing and relative motion within the assembly itself tend to occur. If either of these movements is hampered, the resultant stresses can crack the seals or the tubesheet and/or break some fibers (usually just below the tubesheet).
It is not apparent from the literature (including patents) on the subject of high temperature batteries, that the preceding problem has been heretofore recognized. Neither has any method of constructing such batteries been described which would inherently avoid or remedy the problem.